Different exhaust gas recirculation systems in internal combustion engines, called EGR systems, are known in the current state of the art.
These systems recirculate exhaust gases from the exhaust manifold to the intake manifold of the engine after subjecting them to a cooling process for the purpose of reducing the amount of NOx emissions.
The cooling process is carried out in heat exchangers formed by cooling chambers housing a group of pipes through which the gas passes that are surrounded by a coolant undergoing permanent recirculation.
Single-pass heat exchangers in which the exhaust gas enters at one end, is distributed among said pipes and exits at the opposite end at a lower temperature after having yielded heat to the coolant, are well known in the art.
These exchangers can include bypass lines allowing the recirculation of exhaust gases without passing through the heat exchanger, under the control of a valve channeling the exhaust gases either towards the heat exchanger or towards the bypass line, according to pre-established conditions.
The capacities of a heat exchanger for an EGR system are defined by 2 parameters:                Efficiency: This is the ratio of the obtained cooling and maximum cooling that could be obtained under working conditions:Ef=(Tig−Tog)/(Tig−Tiw), where        Ef=efficiency        Tig=inlet gas T        Tog=outlet gas T        Tiw=inlet water or coolant T        Pressure drop. This is the loss of pressure in the gas due to friction, changes of section and other turbulences that the gas experiences while traveling through the part.        
In all heat exchangers for an EGR system efficiency tends to be maximized so as to thus reduce the level of NOx produced in the engine and to minimize the pressure drop for the purpose of being able to recirculate the largest amount of exhaust gas.
When designing a heat exchanger for an EGR system, it is also necessary to take into account the available space in the engine, so a given length in each case cannot be exceeded for the purpose of improving the efficiency of the part.
In this sense, two-pass heat exchangers for an EGR system are known which have a rounded head at one of their ends, forcing the gas to re-enter the pipes subjected to cooling, so that the gas carries out two passes through them, hence the name.
In this type of exchangers the gas inlet has the outlet attached, and it further allows incorporating a bypass valve to bypass the heat exchanger during the first few minutes after starting up the engine so as to aid it to quickly reach the operating temperature and to start up the catalyst.
The two-pass heat exchanger is more efficient than the one-pass heat exchanger, although the pressure drop is somewhat greater as well (depending on the number of pipes used) and the outer diameter of the casing is larger. However, a casting piece must be used at the inlet, separating the inlet from the outlet, notably making it more expensive.
However, if the outlet of the exhaust manifold from where the EGR gas is taken is located at one end of the exchanger and the inlet to the intake manifold is at the opposite end (where the gas must be taken to after making it pass through the exchanger), it will be necessary on multiple occasions to add an external pipe so as to carry the cooled gas to the point of destination.
The need to use this external pipe complicates the designs due to the lack of space in most engines, and on many occasions making the use of this type of exchangers unfeasible.
The automotive industry demands improvements in known EGR systems so as to respond to different needs. One of them has been brought about by the growing demands of administrative regulations regarding admissible NOx emission levels. Another need that must be met is that of facilitating the assembly of engines in automobiles by simplifying the design of their components so as to improve the integration capacity.